Space debris is an artificial object which is orbiting in geocentric orbits without being used. Examples of space debris include: spacecrafts (artificial satellites, space stations, space shuttles, and the like) which terminated operations due to end of lives, accidents and failures; bodies and components of rockets used to launch artificial satellites; fragments generated in stage separation of multistage rockets; tools dropped by astronauts during extravehicular activities; and the like. Moreover, the space debris also includes minute debris generated by space debris colliding with each other and being crushed into pieces. Accordingly, there are various sizes of space debris.
Detection of the space debris is important in achieving smooth operations of spacecrafts (artificial satellites, space stations, space shuttles, and the like). One of conventional methods of detecting space debris is a method of identifying existence of space debris by extracting a high-luminance portion in images captured with a CCD camera from the globe or on an orbit.
In this method, from three or more images captured at time intervals, images of regions in the same size are cut out in line with a motion (a vector direction and a scalar quantity) of a moving object such as space debris, and a stacking method is applied to the cut-out images. To be more precise, the cut-out images are stacked on one another and the median value or average value of pixel values is calculated for each pixel of the cut-out images. Then, each pixel having the calculated median value or average value being equal to or above a predetermined value is extracted as a pixel representing existence of the moving object (see PTL 1, for example).